Provided is plasma processing method in which a uniform in-plane distribution from the start to the end of etching can be maintained by optimizing the etching conditions. The plasma processing method comprises a step of etching a substrate (W) having a mask pattern formed on the surface thereof with use of plasma formed in a vacuum tank (21), and a step of forming a protective film on the sidewall of an etching pattern formed on the substrate (W) by sputtering a target material (30) placed in the vacuum tank (21) with use of plasma, in which both of the steps are alternately repeated. According to the plasma processing method, a uniform in-plane distribution from the start to the end of plasma processing can be maintained by changing the radius of a magnetic neutral line (25) according to progress of plasma processing including etching and formation of a protective film for the substrate (W).